Phase Lock Loop (PLL) is a common electronic feedback system used in various electronic systems (e.g., radars, communications, TV, computers, cell phones, etc.) Some of the key parts of a PLL system include a phase detector (PD), a local oscillator (LO), a loop filter, etc. The LO frequency can be tuned by changing the input voltage or current. In a PLL circuit, the LO is forced to follow frequency/phase of an incoming/reference signal. The LO is locked onto the incoming signal such that LO and incoming/reference signal have ‘identical’ frequency/phase performance in the bandwidth of interest.
Optical Phase Lock Loop (OPLL) is the counterpart of an electronic PLL in the optical domain. In an OPLL circuit, two lasers are used. They may be referred as a ‘master’ and ‘slave.’ One laser (typically, the slave laser) plays the role of LO and is locked to another reference signal coming from the other laser (typically, the master laser.) As a result, both the lasers in an OPLL system have identical frequency/phase characteristics in the bandwidth of interest.
The two lasers used in the OPLL circuit may be diode-pumped Nd: YAG lasers or fiber lasers. Such lasers have slow speed of frequency tuning (typically, tens of millisecond range) limited by the speed of piezo-electric transducer (PZT) used to change the cavity length of the lasers. Another disadvantage of lasers is that they are usually highly sensitive to environmental changes, and require frequent calibration and maintenance to achieve desired performance.
An alternative to the Nd: YAG laser and the fiber laser are semiconductor-based devices, such as Distributed Feedback Laser (DFB) and External Cavity Lasers (ECL). Semiconductor-based OPLL devices offer the advantage of smaller size and lower power consumption, and often offer competitive or lower cost of manufacturing. OPLL based on DFB lasers require very large bandwidth (BW) of operation because of the large linewidth of DFB lasers (more than a few hundred kHz). Such large BW of operation consequently includes so-called phase reversal frequency (1-10 MHz), where phase of laser frequency modulation (FM) response changes its sign. Presence of the phase reversal frequency within the BW of OPLL leads to instability in operation and requires complicated electronics for correcting such effects. Another adverse problem with DFB based OPLL is their very high sensitivity of FM response caused by current and temperature response to the environmental changes, which forces the OPLL to “ramp” outside of the voltage locking range, and correspondingly lose their lock. Semiconductor-based ECL technology provides very narrow linewidth (in the range of 10 kHz or less), low frequency noise, and FM bandwidth up to 1 MHz, all of which are essential for stable OPLL implementation. ECLs with very narrow linewidth do not require very large BW and correspondingly allow to avoid operation with phase reversal frequency within the OPLL bandwidth. Inherent wavelength stability of ECL to the environmental changes makes them an excellent candidate for OPLL implementation with ability of long-term locking operation.
Accordingly, what is needed is an implementation of OPLL system using semiconductor-based ECLs and associated electronic components, packaged in a relatively smaller-footprint package that offers reliable and easily controllable performance at low power consumption and manufacturing cost.